The present invention relates to methods and apparatus for detecting transfer leaks in solid polymer electrolyte fuel cells and locating such cells in fuel cell stacks.
Electrochemical fuel cells convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes each comprise an electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions.
Solid polymer fuel cells employ a solid polymer electrolyte, or ion exchange membrane. The membrane is typically interposed between two electrode layers, forming a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d). The membrane is typically proton conductive and acts as a barrier, isolating the fuel and oxidant streams from each other on opposite sides of the MEA. The MEA is typically interposed between two plates to form a fuel cell assembly. The plates act as current collectors, provide support for the adjacent electrodes, and typically contain flow field channels for supplying reactants to the MEA or circulating coolant. The fuel cell assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, as well as good sealing between fuel cell components. A plurality of fuel cell assemblies may be combined electrically, in series or in parallel, to form a fuel cell stack. In a fuel cell stack, a plate may be shared between two adjacent fuel cell assemblies, in which case the plate also separates the fluid streams of the two adjacent fuel cell assemblies. Such plates are commonly referred to as bipolar plates and may have flow channels for directing fuel and oxidant, or a reactant and coolant, on each major surface, respectively.
The fuel fluid stream which is supplied to the anode may be a gas such as, for example, substantially pure gaseous hydrogen or a reformate stream comprising hydrogen, or a liquid such as, for example, aqueous methanol. The fuel fluid stream may also contain other fluid components such as, for example, nitrogen, carbon dioxide, carbon monoxide, methane, and water. The oxidant fluid stream, which is supplied to the cathode, typically comprises oxygen supplied as, for example, substantially pure gaseous oxygen or a dilute oxygen stream, such as, for example, air, which may also contain other components such as nitrogen, argon, water vapor, carbon monoxide, and carbon dioxide. Various sealing mechanisms are used to fluidly isolate the fuel and oxidant streams from one another in the fuel cell.
The electrochemical reactions in a solid polymer fuel cell are generally exothermic. Accordingly, a coolant is typically also needed to control the temperature within a fuel cell assembly to prevent overheating. Conventional fuels cells employ a liquid, such as, for example, water to act as a coolant. In conventional fuel cells, the coolant stream is fluidly isolated from the reactant streams.
Thus, conventional fuel cells typically employ three fluid streams, namely fuel, oxidant, and coolant streams, which are fluidly isolated from one another. See U.S. Pat. No. 5,284,718 and FIGS. 1, 2A and 2B of U.S. Patent No. 5,230,966, for examples of typical fuel cell assemblies configured to fluidly isolate the aforesaid three fluid streams. Each of the foregoing ""718 and ""966 patents is incorporated herein by reference in its entirety. Fluid isolation is important for several reasons. For example, one reason for fluidly isolating the fuel and oxidant streams in a hydrogen-oxygen fuel cell is that hydrogen and oxygen are particularly reactive with each other. Accordingly, in solid polymer fuel cells an important function of the membrane and plates is to keep the fuel supplied to the anode separated from the oxidant supplied to the cathode. The membrane and plates are, therefore, substantially impermeable to hydrogen and oxygen. However, since the membrane also functions as an electrolyte, the membrane is generally permeable to protons and water. (Water is generally required for proton transport in membrane electrolytes.)
The coolant fluid is preferably isolated from the reactant fluids to prevent dilution and contamination of the reactant streams. Furthermore, in a conventional fuel cell, it is undesirable to mix a liquid coolant, such as water, with a gaseous reactant such as hydrogen or oxygen. Water may cause flooding in the reactant fluid passages, which prevents the reactants from reaching the electrochemically active membrane-electrode interface. It is also undesirable for the reactant streams to leak into the coolant stream because this reduces operating efficiency, as the leaked reactants are not used to generate electrical power. Likewise, leakage of any of the fluids to the surrounding atmosphere is generally undesirable.
There are several conventional methods of detecting leaks. For example, in a hydrogenoxygen fuel cell, the oxidant exhaust stream can be monitored to detect the presence of hydrogen. When hydrogen is detected in the oxidant exhaust stream, this may indicate a leak. A problem with this method is that hydrogen may be present in the oxidant exhaust stream for reasons other than a leak. For example, if there is a shortage of oxygen at the cathode, protons arriving at the cathode from the anode may recombine with electrons to form hydrogen. There are many possible causes for such an oxygen shortage. For example, an oxygen shortage may result from a sudden increase in power output demand, a malfunctioning compressor, a blockage in fluid flow field channels caused by an accumulation of product water, or a clogged air filter. An oxygen shortage may result in complete or partial oxygen starvation resulting in a reduction in cell voltage and the production of hydrogen by the recombination of protons with electrons on the cathode side of the fuel cell.
An additional problem with using a constituent such as hydrogen, other reactants, or reaction products, as an indicator of a leak is that these constituents may be reactive within the fuel cell. These constituents may be particularly reactive in the presence of the electrocatalyst at the interfaces between the electrolyte and the anode and cathode. Consequently, these substances may react partially or completely prior to being exposed to a detector located in the fluid exhaust manifold. Thus, the concentration of any detected substances may not accurately reflect the amount of the constituent substance that is leaking and may delay the detection of a leak.
When the fuel stream comprises carbon dioxide, a method of detecting leaks between the fuel and oxidant fluid streams involves detecting greater than a threshold level of carbon dioxide in the oxidant exhaust stream. A disadvantage of this method is that an oxidant supply stream, such as air, may already comprise carbon dioxide in varying concentrations. This may be especially true in vehicular applications where the oxidant intake may receive air comprising the exhaust streams of other vehicles. Therefore, a disadvantage of this method is that, for reliable operation, it is necessary to measure the carbon dioxide concentration in the oxidant intake stream, as a reference, in addition to measuring the carbon dioxide concentration in the oxidant exhaust stream.
Another method of detecting leaks between the fuel and oxidant fluid streams is to measure the oxygen concentration in the fuel exhaust stream. Like the aforementioned methods, a problem with this method is that there are other potential sources of oxygen at the anode. For example, sometimes oxygen is introduced into fuel reformate supply streams to counter the effects of catalyst poisoning. Another source of oxygen at the anode is water that may be converted to oxygen, electrons, and protons at the anode when there is a shortage of fuel (which is referred to as fuel starvation). Therefore, a disadvantage of these oxygen detection methods is that other parameters must be analyzed to determine when the oxygen measured within the fuel exhaust stream is the result of fuel starvation, a leak, or residual oxygen that was added to the fuel supply stream.
Fuel cells are also typically checked for leaks prior to operating the fuel cell to produce power, for example, after assembly or during routine maintenance. Another method of checking for leaks is to introduce a gas into the inlet of one of the fluid passages while the outlet is sealed. The other fuel cell fluid passage inlets are sealed and the outlets are typically fluidly connected to a bubble tube. The volume of any gas that bubbles through the bubble tube is measured to determine if there is any leakage. A problem with this test is that it is difficult to administer with consistent results and the test is a time consuming one. Also, particularly with respect to the reactant fluid passages, the pressurization of only one reactant fluid passage may result in damage to the thin membrane electrolyte layer and/or other fuel cell components.
Further, in the foregoing methods it may not be possible to easily and reliably determine which fuel cell(s) in the stack are leaking, if a leak is detected. Thus, it is often necessary to disassemble the stack and re-test it in sections until each leaking cell is found. Such a time-intensive, iterative process is less than desirable.
Accordingly, there is a need for a simple and reliable method of detecting a leak in a fuel cell stack. There is a need for a method that provides a rapid indication of a leak and that can identify the fuel cell or cells that are leaking.
A method of detecting transfer leaks within a fuel cell stack is provided. The stack comprises a plurality of solid polymer electrolyte fuel cell assemblies, at least one fuel manifold fluidly connected thereto, and at least one oxidant manifold fluidly connected to the fuel cell assemblies. In one embodiment the method comprises:
(a) supplying fuel to the fuel manifold(s) at a first pressure;
(b) supplying oxidant to the oxidant manifold(s) at a second pressure; and
(c) measuring the voltage across at least one of the fuel cell assemblies.
The first pressure may be greater than or equal to the second pressure, or the first pressure may be less than the second pressure.
Another embodiment of the present method comprises:
(a) supplying fuel to the fuel manifold(s) at a first pressure;
(b) supplying an inert gas to the oxidant manifold(s) at a second pressure lower than the first pressure; and
(c) measuring the voltage across at least one of the fuel cell assemblies.
In yet another embodiment of the present method, the fuel cell assemblies further comprise a membrane electrode assembly comprising an anode, a cathode, and an ion exchange membrane interposed therebetween, and the fuel cell stack further comprises coolant stream passages in thermal communication with at least a portion of the fuel cell assemblies and at least one coolant manifold fluidly connected to the coolant stream passages.
The embodiment of the present method comprises:
(a) supplying fuel to the fuel manifold(s) at a first pressure;
(b) supplying oxidant to the oxidant manifold(s) at a second pressure;
(c) supplying a gas to the coolant manifold(s) at a third pressure; and
(d) measuring the voltage across at least one of the fuel cell assemblies,
wherein the gas is selected from the group consisting of oxidant and an inert gas, and wherein the third pressure is greater than the first pressure, the second pressure, or both. The first pressure or the second pressure, but not both, may be greater than or equal to the third pressure.
In yet another embodiment, the present method comprises:
(a) supplying oxidant to the oxidant manifold(s) at a first pressure;
(b) supplying a gas to the fuel manifold(s) at a second pressure;
(c) supplying fuel to said at least one coolant manifold(s) at a third pressure; and
(d) measuring the voltage across at least one of the fuel cell assemblies,
wherein the gas is selected from the group consisting of fuel, oxidant and an inert gas, and wherein the third pressure is greater than the first pressure, the second pressure, or both. The first pressure may be greater than or equal to the third pressure. Where the gas is fuel, the second pressure may be greater than or equal to the third pressure.
Yet another embodiment of the present method comprises:
(a) supplying a first gas to the oxidant manifold(s) at a first pressure;
(b) supplying a second gas to the fuel manifold(s) at a second pressure;
(c) supplying fuel to the coolant manifold(s) at a third pressure; and
(d) measuring the voltage across at least one of the fuel cell assemblies,
wherein the first gas is an inert gas and the second gas is selected from the group consisting of fuel and an inert gas, and wherein the third pressure is greater than the first pressure, the second pressure, or both. The first pressure or the second pressure, but not both, may be greater than or equal to the third pressure.
A further embodiment of the present method comprises:
(a) supplying an inert gas to the oxidant manifold(s) at a first pressure;
(b) supplying fuel to the fuel manifold(s) at a second pressure;
(c) supplying oxidant to the coolant manifold(s) at a third pressure; and
(d) measuring the voltage across at least one of the fuel cell assemblies,
wherein the third pressure is greater than the first pressure, the second pressure, or both. The first pressure or the second pressure, but not both, may be greater than or equal to the third pressure.
Yet another embodiment of the present method comprises:
(a) supplying fuel to the coolant manifold(s) at a first pressure;
(b) supplying a gas to the oxidant manifold(s) at a second pressure; and
(c) measuring the voltage across at least one of the fuel cell assemblies,
wherein the gas is selected from the group consisting of oxidant and an inert gas. The first pressure may be greater than or equal to the second pressure.
Still another embodiment of the present method comprises:
(a) supplying oxidant to the coolant manifold(s) at a first pressure;
(b) supplying fuel to the fuel manifold(s) at a second pressure; and
(c) measuring the voltage across at least one of the fuel cell assemblies.
The first pressure may be greater than or equal to the second pressure.
In the present method, the stack may be operating at open circuit voltage. The method may also further comprise comparing the measured voltage with a reference voltage.
The pressure differentials between the various gases supplied to the stack may be any suitable pressure differential. For example, the pressure differential may be between 0 kPa and about 200 kPa, and preferably may be between about 6.5 kPa and about 35 kPa.
An apparatus for detecting transfer leaks within a fuel cell stack is also provided. In one embodiment, the present apparatus comprises:
(a) a fuel source fluidly connected to the fuel manifold(s) for supplying fuel thereto at a first pressure;
(b) a gas source fluidly connected to the oxidant manifold(s) for supplying a gas thereto at a second pressure; and
(c) at least one device for measuring the voltage across at least one of the fuel cell assemblies,
wherein the gas supplied to the oxidant manifold(s) is selected from the group consisting of oxidant and inert gas.
In another embodiment, the present apparatus comprises:
(a) a fuel source fluidly connected to the fuel manifold(s) for supplying fuel thereto at a first pressure;
(b) an oxidant source fluidly connected to the oxidant manifold(s) for supplying oxidant thereto at a second pressure;
(c) a gas source fluidly connected to the coolant manifold(s) for supplying a gas thereto at a third pressure; and
(d) at least one device for measuring the voltage across at least one of the fuel cell assemblies,
wherein the gas supplied to the coolant manifold(s) is selected from the group consisting of oxidant and an inert gas. The oxidant source and first gas source may be the same or different.
Yet another embodiment of the present apparatus comprises:
(a) an oxidant source fluidly connected to the oxidant manifold(s) for supplying oxidant thereto at a first pressure;
(b) a gas source fluidly connected to the fuel manifold(s) for supplying a gas thereto at a second pressure;
(c) a fuel source fluidly connected to the coolant manifold(s) for supplying fuel thereto at a third pressure; and
(d) at least one device for measuring the voltage across at least one of the fuel cell assemblies,
wherein the gas supplied to the fuel manifold(s) is selected from the group consisting of fuel, oxidant and inert gas. Where the gas is fuel, the fuel source and gas source may be the same or different. Similarly, where the gas is oxidant the oxidant source and gas source may be the same or different.
A further embodiment of the present apparatus comprises:
(a) a first gas source fluidly connected to the oxidant manifold(s) for supplying a first gas thereto at a first pressure;
(b) a second gas source fluidly connected to the fuel manifold(s) for supplying a second gas thereto at a second pressure;
(c) a fuel source fluidly connected to the coolant manifold(s) for supplying fuel thereto at a third pressure; and
(d) at least one device for measuring the voltage across at least one of the fuel cell assemblies,
wherein the first gas is an inert gas and the second gas is selected from the group consisting of fuel and inert gas. Where the second gas is fuel, the fuel source and second gas source may be the same or different. Similarly, where the second gas is an inert gas, the first gas source and second gas source may be the same or different.
A still further embodiment of the present apparatus comprises:
(a) an inert gas source fluidly connected to the oxidant manifold(s) for supplying inert gas thereto at a first pressure;
(b) a fuel source fluidly connected to the fuel manifold(s) for supplying fuel thereto at a second pressure;
(c) an oxidant source fluidly connected to the coolant manifold(s) for supplying oxidant thereto at a third pressure; and
(d) at least one device for measuring the voltage across at least one of the fuel cell assemblies.
Another embodiment of the present apparatus comprises:
(a) a fuel source fluidly connected to the coolant manifold(s) for supplying fuel thereto at a first pressure;
(b) a gas source fluidly connected to the oxidant manifold(s) for supplying a gas thereto at a second pressure; and
(c) at least one device for measuring the voltage across at least one of the fuel cell assemblies,
wherein the gas supplied to the oxidant manifold(s) is selected from the group consisting of oxidant and inert gas.
Yet another embodiment of the present apparatus comprises:
(a) an oxidant source fluidly connected to the coolant manifold(s) for supplying oxidant thereto at a first pressure;
(b) a fuel source fluidly connected to the fuel manifold(s) for supplying fuel thereto at a second pressure; and
(c) at least one device for measuring the voltage across at least one of the fuel cell assemblies.
In the present apparatus, the device for measuring the voltage across at least one of the fuel cell assemblies may be removably attached to the stack. Further, the device may generate an output signal representative of the measured voltage, in which case the apparatus may further comprise a voltage display for receiving the output signal from the device.
A further embodiment of the present apparatus comprises: a device(s) for measuring the voltage across at least one of the fuel cell assemblies of a fuel cell stack, and for generating an output signal representative of the measured voltage; and, a voltage display for receiving the output signal from the device(s). The stack may be adapted to receive the device(s), and the device(s) may be removably attachable to the stack. The apparatus may further comprise at least one connector for fluidly connecting a fuel source, an oxidant source, or both, to at least one of the fuel, oxidant, and coolant manifolds of the stack. The embodiment may optionally further comprise an inert gas source fluidly connected to the fuel manifold, the oxidant manifold, or both, of the stack.
In the present method and apparatus, the inert gas may be selected from the group consisting of nitrogen, argon, helium, and carbon dioxide.